The present invention relates to apparatus for coating a liquid composition onto a substrate surface to form a coating thereupon and a method for determining the physical parameters of and making said hopper and, more particularly to a hopper for coating one or more liquid compositions from a plurality of distribution slots onto a substrate surface to form a coating, and most particularly to a multiple-slide hopper having a plurality of internal distributional cavities, internal slots, distribution slots, slide surfaces, and offsets between slide surfaces wherein the shapes of and relationships among these elements are optimized to permit transversely-uniform coatings of compositions having a wide range of Newtonian and shear-thinning Theological properties.
In forming a flowing sheet of a liquid composition for coating onto a substrate surface, the composition is reshaped typically from collimated flow in a pipe to sheet flow for application by an apparatus known variously in the art as a die, a distributor, an extruder, and a hopper. As used herein, all such types of apparatus are referred to collectively as hoppers. A hopper may comprise one or more parallel longitudinal members which are oriented transversely of the direction of liquid flow, which members may be bolted together or otherwise attached to form a hopper unit. A primary member may be referred to as a xe2x80x9chopper body,xe2x80x9d and one or more secondary members asxe2x80x9chopper bars.xe2x80x9d Within a hopper, a flow path for liquid composition typically includes an inlet, one or more transverse distributional voids known as cavities or channels, and a slotted exit from each cavity communicating with either a successive cavity or the exterior of the hopper. The last such slot in the flow path is commonly known as an exit slot.
In extrusion/slide hoppers, as are used typically in the manufacture of photographic films and papers, composition is extruded upwards from the exit slot onto an inclined slide surface terminating at a lower edge in a coating lip. The extruded sheet flows down the slide surface under gravity and is transferred to the passing substrate either through a dynamic longitudinal bead, as in bead coating, or a falling curtain, as in curtain coating.
It is well known that a hopper unit may combine a plurality of individual distribution systems to permit the simultaneous application of a plurality of compositions and/or the split flows of a single composition having an exceptionally high flow rate (see, for example, U.S. Pat. No. 5,143,758 issued Sep. 1, 1992 to Devine). Such a hopper is known in the art as a multiple-slot hopper. The full stack of compositions to be coated is assembled as the liquids flowing from the exit slots of the individual distribution systems progressively become stacked on the hopper slides, each additional liquid sheet becoming the bottom-layer conveyance for the already-assembled stack of sheets sliding onto it from higher on the inclined hopper surface.
It is known that each slide surface is preferably offset vertically from (generally slightly lower than) the surface of the next slide farther from the substrate, to accommodate the new flow being added to the bottom of the stack. If an offset is too small or too large, the layers may not join smoothly; in particular, eddies and bubble traps may form that can result in nonuniformly coated layers. In the extreme, the sliding layers can form stable longitudinal ridges at an offset that is too high. Means for determining the optimum offset to accommodate a wide range of Newtonian and non-Newtonian rheologies has not heretofore been disclosed.
The slide surface of an extrusion/slide hopper terminates immediately above the uppermost slot in a wall, or xe2x80x9cbackland,xe2x80x9d for attachment of the free upper surface of the coating pack. It is known that if the backland is too high, attachment may occur irregularly along the vertical face of the wall and cause streaks; moreover, the upwardly curved meniscus traps bubbles. If the backland is too low, such attachment occurs irregularly along the hopper surface above the backland. A proper height backland permits the upper layer to attach uniformly at the well-defined upper edge of the backland while minimizing the risk of trapping bubbles. Typically, backlands may be about 2.5 mm high; however, means for determining the optimum backland height have not heretofore been disclosed.
It is standard practice in the coating art to fill all the slots in a hopper in use. Thus, for example, if a four-composition coating is to follow a five-composition coating using a five-slot hopper, either a four-slot hopper must be substituted between coatings or one of the compositions in the second coating must be split and delivered through two adjacent slots (See U.S. Pat. No. 5,143,758 supra). The former alternative requires the building of two entirely separate, expensive hopper units. In a large manufacturing practice, a large fleet of hoppers having different numbers of delivery slots may be required, which can be very expensive to fabricate and maintain. The latter alternative can require undesirably low flow rates through some slots or undesirable dilution of layers to artificially increase flow rates.
An alternative, not successfully practiced heretofore, is to use only the number of slots that is optimal for the compositions to be coated. In the above example, the fifth slot of the five-slot hopper could be left empty for the second coating if advantageous. A serious practical problem arises in so doing, however, as the fifth/fourth slide offset must now function as the backland, and typically such offset is substantially smaller than that conventionally used for a backland. Such a hopper, therefore, is not properly versatile.
Means for determining an acceptable height for an offset which can function either as a flow offset within a coating stack or as a backland at the top of a hopper slide, to increase the versatility of a hopper, has not heretofore been disclosed.
Distribution arrangements for each flow within a hopper typically include a flared central inlet (see, for example, U.S. Pat. No. 5,256,052 issued Oct. 26, 1993 to Cloeren) connecting a feed pipe to the center of a generally bilaterally-symmetrical first distributional cavity disposed transversely of the required sheet flow and web conveyance direction. A first slot connects the first cavity with a second cavity generally parallel with the first cavity. A second slot connects the second cavity with the slide surface of the hopper. Such a hopper may be referred to in the art as a xe2x80x9cdual cavityxe2x80x9d hopper having a primary cavity and slot and a secondary cavity and slot.
The functions of these hopper flow elements are as follows. The inlet is flared downstream where it joins the first cavity to begin the conversion of composition flow from flow in a conduit to cavity flow transversely of the direction of coating. The first or primary cavity provides the initial and principal widthwise distribution of composition. Because velocity is lost as the liquid flows along a cavity of constant cross-sectional area, the primary cavity preferably is tapered in continuously-declining cross-sectional area between the center and the two ends (See, for example, U.S. Pat. No. 4,285,655 issued Aug. 25, 1981 to Matsubara; U.S. Pat. No. 5,234,649 issued Aug. 10, 1993 to Cloeren; and U.S. Pat. No. 5,494,429 issued Feb. 27, 1996 to Wilson et al). The cross-sectional shape may be generally circular, generally rectangular, or a combination of the two (See, for example, U.S. Pat. No. 4,222,343 issued Sep. 16, 1980 to Zimmermann et al.; U.S. Pat. No. 5,256,052 issued Oct. 26, 1993 to Cloeren; U.S. Pat. No. 5,593,734 issued Jan. 14, 1997 to Yuan et al.; and U.S. Pat. No. 5,643,363 issued Jul. 1, 1997 to Hosogaya et al).
The slot adjoining the primary cavity, the primary slot, is small enough in height, typically about 0.025 cm or less, to create a substantial back pressure in the primary cavity. The length of the slot between the cavities may be constant or may be tapered to compensate for pressure loss along the primary cavity. The primary cavity and slot cooperate to provide into the secondary cavity a flow of composition in the coating direction that is nearly uniform in flow per unit of coated width.
The secondary cavity permits smoothing of the widthwise pressure gradient such that flow pressure presented to the secondary slot is highly uniform across the entire width of the slot. The secondary slot has a precisely uniform length, and so the flow distribution is highly uniform. Preferably, the secondary cavity is configured to smooth transverse pressure gradients without formation of eddies or stagnant regions within the cavity (See, for example, U.S. Pat. No. 5,234,500 issued Aug. 10, 1993 to Korokeyi, the disclosure of which is hereby incorporated by reference).
The secondary slot has a uniform length between the secondary cavity and the slide surface and a uniform height, for example, 0.025 cm. The slot exit may be simply the end of the slot itself, or it may be an expansion (for example, U.S. Pat. No. 3,005,440 to Padday).
Means for determining the combined optimum parameters for the primary cavity, primary slot, secondary cavity, and secondary slot have not heretofore been disclosed.
It is known in single-slot hoppers to adjust the height of a slot, as by bending the bars using adjusting bolts, to fine-tune flow uniformity therefrom. Such tuning is not practically possible in multiple-slot hoppers, because individual bars are not accessible for loading and because mechanical distortion affects more than one slot.
Also, it is known in single-slot hoppers to specialize the internal design for specific rheological and flow conditions. This stratagem is not attractive for multiple slot hoppers. Therefore, in known multiple-slot hoppers, typically the same parameters are used for designing and fabricating each flow system so that all individual distribution systems are geometrically identical. This is attractive because of the wide range of flow conditions that each system must accommodate in day-to-day operation. It has been possible, of course, to design and fabricate each distribution system to be optimum for a specific flow rate of a specific coating composition, but such a hopper then becomes dedicated to use with a particular combination of compositions (a product). Such specialization is very expensive for each custom hopper, and back-up equipment can be provided only at double the expense. For an establishment that manufactures many coated products having significantly different numbers of compositions having differing rheological properties and flow rates, such specialization is clearly not feasible.
Therefore, hopper flow systems typically are designed to accommodate some ranges of rheological properties and flow rates. In recent years, the range of conditions that needs to be accommodated has expanded considerably; the reasons include increases in coating speed and more severely shear-thinning coating compositions. At one extreme, hoppers for photographic products must accommodate Newtonian rheology; aqueous gelatin, the common vehicle for photographic compositions, is Newtonian or nearly so in hoppers. At the other extreme, coating compositions can be significantly shear thinning; that is, viscosity decreases with an increasing rate of shearing. Photographic compositions contain emulsions and dispersions, and such colloids impart shear thinning behavior as they are concentrated, as by a reduction in the solvent (water) to extend the capabilities of existing dryers. Viscosity enhancing agents, usually called thickeners, are also increasingly used. Most often these are polymers of high molecular weight that physically crosslink with gelatin molecules, and they almost always promote shear thinning. Thus, known hoppers lack adequate versatility.
A significant restriction in hopper design is the total height of the bars. Existing coating stations may severely restrict an increase in bar height to accommodate a more versatile design. In curtain coating, bar height is usually restricted by curtain height. In any case, taller bars increase hopper weight making transport and positioning more difficult. Taller bars also increase the leverage of internal liquid pressures for bending the bars and compromising slot height. Taller bars may have to be thicker to increase their stiffness, but thicker bars increase hopper weight and the length of the hopper slide. As superimposed layers flow down the hopper slide, waves arise because of mechanical vibrations and flow-rate pulsations (for example, U.S. Pat. No. 5,376,401). Flow down the slide may amplify any such waves, and the longer the slide the greater the amplification. Thus, it is known to minimize slide length, and thicker bars oppose that aim.
Thus there is a need for a method to determine, for a wide range of flow rates and Newtonian and shear-thinning Theological conditions, the optimum dimensions of elements of a versatile dual-cavity extrusion/slide hopper, including primary cavity shape and size in three dimensions, primary slot height, length and taper, secondary cavity shape and size in three dimensions, secondary slot height and length, and slide offset at a slot exit. The versatility cannot come at the expense of a large increase in bar height or thickness.
It is a principal object of the invention to provide a method for determining a versatile, dual-cavity, multiple-slot extrusion/slide hopper.
It is a further object of the invention to provide an improved multiple-slot hopper wherein uniformity of liquid composition delivery from each slot is maximized in the widthwise direction.
It is a still further object of the invention to provide an improved multiple-slot hopper wherein the parameters and consequent physical dimensions of all composition distribution systems are identical.
It is a still further object of the invention to provide versatility without greatly increasing bar height, hopper weight, or slide length.
It is a still further object of the invention to provide an improved hopper wherein each slide offset can function either as a flow offset within a coating stack or as a suitable backland at the top of a coating stack.
The invention is defined by the claims. The apparatus and method of the invention are useful in providing highly uniform coatings of liquid compositions to moving webs over a wide range of flow rates and of Newtonian and shear-thinning rheologies.
Briefly described, the invention includes a versatile, multiple-slot, dual cavity, slide/extrusion hopper for assembling and coating a highly uniform composite layer comprising a plurality of superposed compositions, and a method for designing the hopper such that each extruded flow is highly uniform in thickness, slide offsets can function as flow offsets within the coating stack or as a suitable backland at the top of the utilized slide, each slide offset is sized to prevent eddies and bubble trap formation, and all individual flow systems within the hopper are geometrically identical. The method provides the optimum cavity cross-sectional shape and rate of taper for the primary cavity, the optimum height, length, and rate of taper for the primary slot, the optimum cavity cross-sectional shape for the secondary cavity, and the optimum height and length for the secondary slot, and the optimum slide offset height. A formula is provided which gives a straightforward and useful way to scale versatile designs and thereby eliminate much experimentation and design effort and reduce the number of coating hoppers required.
The method for determining the physical parameters of flow controlling elements within a coating hopper comprises the steps of:
a) specifying values for rheological parameters of the most shear thinning liquid composition to be dispensed according to the truncated power-law model, including the viscosity at low shear rates (xcexcl), the viscosity at high shear rates (xcexch), the power law index (n), the shear rate above which shear thinning occurs ({dot over (xcex3)}l), and the maximum volumetric flow rate per unit coated width (q)of said composition to be supplied to one of said elements, said rheological parameters conforming to the limits nxe2x89xa70.6 and {dot over (xcex3)}1xe2x89xa7100 per sec;
b) specifying values of the following physical parameters: the half-width of the hopper cavities (L), the initial length of the primary slot (lp) from about 0.5 to about 2 cm, the initial length of the secondary slot (ls) from about 1.0 to about 2.5 cm, the height of the primary slot (hp) from about 0.018 to about 0.030 cm, the height of the secondary slot ( hs)from about 0.038 to about 0.076 cm, the exponent by which the cross section of the primary cavity is tapered (t) from about 0.4 to about 0.6; the primary design factor (Ep) from about 0.005 to about 0.1, the secondary design factor (Es) from about 0.1 to about 0.3, the aspect ratio of the cross section of the primary cavity (rp) from about 1 to about 2 and determining the ratio of the area to the square of the flow length (xcex2p) for said cross section of said primary cavity, the aspect ratio of the cross section of the secondary cavity (rs) from about 2 to about 7 and determining the ratio of the area to the square of the flow length (xcex2p) for said cross section of said secondary cavity;
c) calculating from said specified physical and said rheological parameters the values of the primary viscosity ratio (fp) where             f      p        =                                        6            ⁢                          q                              1                -                n                                                                                        γ                .                            l                              1                -                n                                      ⁢                          h              p                              2                ⁢                                  (                                      1                    -                    n                                    )                                                                    ⁡                  [                      n                          2              ⁢                              (                                                      2                    ⁢                    n                                    +                  1                                )                                              ]                    n        ,
and the secondary viscosity ratio (fs) where             f      s        =                                        6            ⁢                          q                              1                -                n                                                                                        γ                .                            l                              1                -                n                                      ⁢                          h              s                              2                ⁢                                  (                                      1                    -                    n                                    )                                                                    ⁡                  [                      n                          2              ⁢                              (                                                      2                    ⁢                    n                                    +                  1                                )                                              ]                    n        ;
d) calculating values for the remaining of said physical parameters using values obtained in steps a) through c), said remaining physical parameters including the primary geometry ratio (gp) where gp=(1xe2x88x92t)Ep/(fpxe2x88x921) and gpxe2x89xa60.07(1xe2x88x92t), the secondary geometry ratio (gs) where gs=Es/fs, the initial cross-sectional area of the primary cavity (Ap) where Ap={square root over (hp3+L L2+L /lp+L gp+L )}, the initial flow length of the primary cavity (bp) where bp={square root over (Ap+L /xcex2p+L )}, the fractional tapering of the primary slot (m) where m=1xe2x88x92gp(1+fp)/1.5(1xe2x88x92t), the cross-sectional area of the secondary cavity (As) where As={square root over (hs3+L L2+L /ls+L gs+L )}, and the flow length of the secondary cavity (bs) where bs={square root over (As+L /xcex2s+L )}.
The coating hopper of the present invention dispenses a liquid composition to coat a moving substrate. The coating hopper comprises a primary cavity, a primary slot exiting the primary cavity and communicating with a secondary cavity, and a secondary slot exiting the secondary cavity, wherein, preferably, the primary slot height hp is between about 0.018 cm and 0.030 cm, the secondary slot height is between about 0.038 cm and 0.076 cm, the primary slot length is between about 0.5 cm and 2 cm, the secondary slot length is between about 1 cm and 2.5 cm, the a tapering factor t is between about 0.4 and 0.6, the aspect ratio of the primary cavity (rp) is between about 1 and 2, the aspect ratio of the secondary cavity is between about 2 and 7, the primary geometry ratio (gp) is between about 0.001 and 0.04, the secondary geometry ratio (gs) is between about 0.03 and 0.3, and the fractional tapering of the primary slot (m) is between 0.9 and 1.